This application relates in general to the refining of steel, and more particularly to the surface pneumatic refining of carbon or low alloy steels wherein the temperature of the melt is controlled during refining in order that the desired tap temperature be obtained at the end of the refining period.
Several subsurface pneumatic steel refining processes are known in the art including, for example, the AOD, CLU, OBM, Q-BOP and the LWS processes. U.S. patents illustrative of these processes, respectively, are: U.S. Pat. Nos. 3,252,790, 3,867,135, 3,706,549, 3,930,843 and 3,844,768.
The term "subsurface pneumatic refining" as used in the present specification and claims is intended to mean a process wherein decarburization of the melt is achieved by the subsurface injection of oxygen gas, alone or in combination with one or more gases selected from the group consisting of argon, nitrogen, ammonia, steam, carbon dioxide, hydrogen methane or higher hydrocarbon gas. The gases may be blown in by following various blowing programs depending on the grade of steel made and on the specific gases used in combination with oxygen. In addition to decarburization, subsurface pneumatic refining may also cause the melt to become desulfurized, dephosphorized and degassed. Furthermore, the refining period may end with certain finishing steps, such as lime and alloy additions to reduce the oxidized alloying elements and form a basic slag, and addition of alloying elements to adjust the melt composition to meet melt specifications.
The melt is heated by the exothermic oxidation reactions which take place during the decarburization stage of the refining period, while the melt cools quite rapidly during the finishing stage since the additions of lime and alloying elements are endothermic, as well as the fact that no exothermic reactions are taking place.
Subsurface pneumatic refining, commonly referred to in the art as "blowing", normally produces one or more of the following results: decarburization, deoxidation, desulfurization, and degassing of the heat. In order to obtain these results it is necessary to provide sufficient oxygen to burn out the carbon to the desired level (decarburization), sufficient sparging gas to thoroughly mix the deoxidizing additions into the melt and to achieve good slag-metal interaction (deoxidation), to obtain a basic slag (for desulfurization), and sufficient sparging gas to assure that low levels of hydrogen and nitrogen will be obtained in the melt (degassing).
Pneumatic refining has two opposing temperature constraints. One is that a sufficiently high temperature must be obtained by the exothermic reactions to permit the endothermic steps to be carried out while maintaining the temperature of the melt sufficiently high for tapping of the heat. The opposing restraint is that the peak temperature attained in the refining vessel must be held below one that will cause excessive deterioration of the refractory lining of the vessel.
All of the above-mentioned surface pneumatic refining processes suffer from the common difficulty of achieving complete refining of the melt while maintaining a sufficiently high temperature to permit tapping of the heat at the end of the refining period. In order to overcome this problem, it is common practice in the art to reblow the heat with oxygen, thereby generating heat by the exothermic oxidation of carbon and metallic elements in the melt.
Although the present invention is applicable to all of the above-mentioned subsurface pneumatic steel refining processes, for purposes of convenience, the invention will be described and illustrated by reference to the argon-oxygen decarburization (AOD) process.
The basic AOD refining process is disclosed by Krivsky in U.S. Pat. No. 3,752,790. An improvement on Krivsky relating to the programmed blowing of the gases is disclosed by Nelson et al in U.S. Pat. No. 3,046,107. The use of nitrogen in combination with argon and oxygen to achieve predetermined nitrogen contents is disclosed by Saccomano et al in U.S. Pat. No. 3,754,894. A modification of the AOD process is also shown by Johnsson et al in U.S. Pat. No. 3,867,135 which utilizes steam or ammonia in combination with oxygen to refine molten metal.
By use of the term "argon-oxygen decarburization or AOD process" in the present specification and claims is meant, a process for refining molten metals and alloys contained in a refining vessel provided with at least one submerged tuyere, comprising (a) injecting into the melt through said tuyere(s) an oxygen-containing gas containing up to 90% of a dilution gas, said dilution gas functioning to reduce the partial pressure of the carbon monoxide in the gas bubbles formed during decarburization of the melt and/or to alter the feed rate of oxygen to the melt without substantially altering the total injected gas flow rate, and thereafter (b) injecting a sparging gas into the melt through said tuyere(s) said sparging gas functioning to remove impurities from the melt by degassing, deoxidation, volatilization, or by flotation of said impurities with subsequent entrapment or reaction with the slag. Optionally, said process may have the oxygen-containing gas stream surrounded by an annular stream of a protective fluid which functions to protect the tuyere(s) and the surrounding refractory lining from excessive wear. The useful dilution gases include argon, helium, hydrogen, nitrogen, carbon monoxide, carbon dioxide, steam or a hydrocarbon gas; argon is preferred. Useful sparging gases include argon, helium, nitrogen and steam; argon being preferred. Useful protective fluids include argon, helium, hydrogen, nitrogen, carbon monoxide, carbon dioxide, steam or a hydrocarbon fluid; argon again is preferred.
During the refining period the temperature of the melt is influenced by those factors that constitute heat losses and those that constitute heat gains. In the refining vessel heat is required to:
(1) raise the temperature of the melt from its charge temperature to its tap temperature,
(2) dissolve the lime, as well as any alloy, scrap or other additions made during refining,
(3) make up for the heat lost by the melt to its surroundings during the overall refining period (i.e. during inert gas stirring, blowing, reduction and turn downs).
Heat is supplied during the refining period only by the exothermic reactions which take place during refining. These include the oxidation of the carbon (decarburization), silicon and other metallic constituents in the melt (such as iron, chrome, manganese, etc.).
When a heat of steel is refined in a relatively large vessel the heat lost per ton of melt is relatively small. Consequently, the heat gained from the exothermic oxidations of carbon, metallics and silicon tends to balance the heat lost. However, when steel is refined in a small vessel, the magnitude of the heat loss per ton of melt can be so great that the heat produced by oxidation will not balance the heat lost. This results in refined heats whose temperature is below the desired tapping temperature. This problem has been commonly overcome by the prior art by reblowing the heat with an oxygen containing gas to generate more heat and hence to raise the temperature of the melt to the desired tapping temperature. Such reblowing is, however, undesirable because it takes additional time, requires the use of additional oxygen and causes undesirable oxidation of metallic elements in the melt, producing inefficiency in the overall refining operation, and adversely affect the quality of the metal.
It would appear possible at first glance to solve the low tapping temperature problem by increasing the magnitudes of the heat gain factors and/or to decrease the mangitudes of the heat loss factors mentioned above as contributing to the overall heat balance. However, closer examination of this problem will show that this is not practical for small vessels.
If carbon were to be added in order to increase the amount available for oxidation, at constant oxygen blowing rates the heat losses would also increase. In fact, the net effect of oxidizing additional carbon is either no heat gain or a heat loss. Since it is undesirable to lose the metallic elements from the heat, an increase in metallic oxidation is likewise undesirable. Moreover, a sufficient increase in the metallic oxidation of carbon steels and low alloy steels would result in high metal oxide levels in the slag which is detrimental to refractory life.
If silicon were added to increase the amount available for oxidation, there would be a net heat increase during the refining operation. However, the more silicon that is added to the melt, the more lime must be added to the melt in order to neutralize the silicon oxide in the slag. The addition of the extra lime is endothermic. Hence, the net effect is a small and therefore impractical way of increasing the temperature of the melt.
It is known that the addition of aluminum to the melt will generate heat by its oxidation. Furthermore, the use of aluminum has several advantages over silicon for providing heat to the melt. Aluminum requires less oxygen than silicon per unit of heat released, and it requires less lime than does silicon to form a basic desulfurizing slag. Hence, if one were to substitute aluminum for silicon in the melt, a greater net heat increase could be produced. However, the use of aluminum to generate heat causes refractory problems because when a steel melt (which normally contains carbon, manganese, silicon, chromium, nickel and molybdenum) is blown with an oxygen-rich gas mixture, the oxygen will always react with the aluminum first. Hence, if sufficient aluminum is added to generate enough heat to permit subsequent refining, essentially all of it will be oxidized before any carbon, silicon or other metallics are oxidized, resulting in temperatures exceeding those permitted without causing excessive refractory deterioration. In the case of typical refractory materials used in AOD vessels, the peak temperature permitted is approximately 3140.degree. F.